The present invention relates to a device for stacking a large amount of paper sheets which are sequentially driven out of an image forming apparatus.
An electrophotographic copier, facsimile apparatus, printer or similar image forming apparatus is often operated with a stacking device capable of stacking a large amount of paper sheets which are sequentially discharged from the apparatus. A prior device of this kind, i.e., paper stacker has a tray for receiving and stacking paper sheets which are sequentially driven out by a discharge roller. A sensor is associated with the tray and has a feeler which rests on the top of the paper stack on the tray for sensing the level or height of the stack. A driving mechanism moves the tray up and down in response to an output of the sensor, so that the top of the stack may be constantly held at a predetermined height. It is a common practice to provide the sensor with a single feeler which is located in a paper discharging position and at substantially the intermediate between the widthwise opposite edges of paper sheets. This brings about a problem when paper sheets each being curled widthwise are stacked on the tray, the top of the resulting stack being higher at opposite side edges than the center. Specifically, when the side edges of the top of the curled stack becomes higher than the paper discharging position, the leading edge of a paper sheet coming out from the paper discharging position is apt to abut against the opposite edges of the stack to thereby jam the paper feed path. The jam ascribable to the curling of paper sheets frequently occurs in hot and humid environments or when the water content of paper sheets is higher than 8%. Conversely, when the humidity is low or when the water content of paper sheets is less than 6%, a paper sheet becomes a perfect insulator and, therefore, it is apt to gather a charge due to friction. In this condition, it often occurs that the paper sheet adheres to structural members around the path to thereby block the path, resulting in the immediately following paper sheet jamming the path.